It is known to burn, calcine or sinter fine-grain material by heating of preheated material to the final treatment temperature in a rotary kiln. However, this method involves considerable plant costs and careful maintenance, particularly of rotating parts.
It is further known (German Pat. No. 474,727) to burn fine-grain material in a vertical shaft through which the material passes from the top to the bottom and the gas from the bottom to the top, a burning zone equipped with burners being provided in the center portion of the shaft. The upwardly flowing exhaust gases preheat the material while the burnt material is cooled in the lower portion of the shaft by the rising secondary air. Because in such methods the material must move in counterflow relationship to the gases where fine-grain material is involved, it is impossible to use high gas velocities and thus impossible to attain high throughput capacities.
A largely uncontrolled relative movement of material and gas is also present in a further known method (German Pat. No. 498,406) in which the material is blown from the bottom upwardly into a shaft furnace and allowed to drop down in the form of a cloud through the rising hot gases. The hot gases in this case are produced by burners disposed in the lower region of the furnace just above the material injection point. Because in this known method the residence time of the individual particles in the hot zone cannot be controlled exactly, uniform burning of the entire material is not possible.
A method is also known (German Pat. No. 1,218,927) in which fine material preheated in cyclones is introduced into a conduit which leads from a burning chamber to a fine material separator. The fine material however does not come into contact with the zone of highest temperature, i.e. with the actual flame zone. Consequently, with this method the relatively short path over which the hot exhaust gases of the burning chamber are in contact with the fine material does not transmit the high quantity of heat necessary to achieve the final treatment temperature uniformly to the entire material.
Furthermore, a method of treating fine-grain ore is known (German Pat. No. 423,620) in which the mixture of ore dust, air and fuel is set in turbulent motion in a space in front of a reaction chamber and then blown into the reaction space. The ore particles melt in the reaction chamber and are withdrawn from the latter in the molten state. Such a method however easily leads to formation of deposits in the reaction chamber and this involves considerable maintenance work. Moreover, because of the largely undefined turbulent motion of the ore particles in the reaction chamber the thermal treatment of the ore is very irregular.